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Fix tautological clause in iter_impl's iterator category assertion

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The static_assert meant to check the LegacyBidirectionalIterator
named requirement had a first clause comparing
std::bidirectional_iterator_tag to itself, which is always true and
checks nothing; only array_t::iterator was actually being checked,
despite the message claiming object iterators were checked too.
Drop the tautological clause, reword the message to describe what
is actually checked, and note that object_t may use a forward-only
iterator as long as reverse iteration and operator-- are unused.
The check is intentionally not extended to object_t::iterator, since
that would reject object types with forward-only iterators that
compile and work correctly today.

Part of #5708

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
This commit is contained in:
Niels Lohmann
2026-09-30 09:54:59 +02:00
parent 03319da529
commit a9cc4243b7
2 changed files with 10 additions and 6 deletions
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include/nlohmann/detail/iterators/iter_impl.hpp
+5 -3
View File
@@ -60,9 +60,11 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
static_assert(is_basic_json<typename std::remove_const<BasicJsonType>::type>::value,
"iter_impl only accepts (const) basic_json");
// superficial check for the LegacyBidirectionalIterator named requirement
static_assert(std::is_base_of<std::bidirectional_iterator_tag, std::bidirectional_iterator_tag>::value
&& std::is_base_of<std::bidirectional_iterator_tag, typename std::iterator_traits<typename array_t::iterator>::iterator_category>::value,
"basic_json iterator assumes array and object type iterators satisfy the LegacyBidirectionalIterator named requirement.");
// note: only array_t::iterator is checked here; object_t::iterator may be
// a forward-only iterator as long as reverse iteration and operator--
// are never used on it
static_assert(std::is_base_of<std::bidirectional_iterator_tag, typename std::iterator_traits<typename array_t::iterator>::iterator_category>::value,
"basic_json iterator assumes array type iterators satisfy the LegacyBidirectionalIterator named requirement.");
public:
/// The std::iterator class template (used as a base class to provide typedefs) is deprecated in C++17.
single_include/nlohmann/json.hpp
+5 -3
View File
@@ -17811,9 +17811,11 @@ class iter_impl // NOLINT(cppcoreguidelines-special-member-functions,hicpp-speci
static_assert(is_basic_json<typename std::remove_const<BasicJsonType>::type>::value,
"iter_impl only accepts (const) basic_json");
// superficial check for the LegacyBidirectionalIterator named requirement
static_assert(std::is_base_of<std::bidirectional_iterator_tag, std::bidirectional_iterator_tag>::value
&& std::is_base_of<std::bidirectional_iterator_tag, typename std::iterator_traits<typename array_t::iterator>::iterator_category>::value,
"basic_json iterator assumes array and object type iterators satisfy the LegacyBidirectionalIterator named requirement.");
// note: only array_t::iterator is checked here; object_t::iterator may be
// a forward-only iterator as long as reverse iteration and operator--
// are never used on it
static_assert(std::is_base_of<std::bidirectional_iterator_tag, typename std::iterator_traits<typename array_t::iterator>::iterator_category>::value,
"basic_json iterator assumes array type iterators satisfy the LegacyBidirectionalIterator named requirement.");
public:
/// The std::iterator class template (used as a base class to provide typedefs) is deprecated in C++17.